Design and implement a data structure for a Least Frequently Used (LFU) cache.
Implement the LFUCache class:
LFUCache(int capacity)Initializes the object with thecapacityof the data structure.int get(int key)Gets the value of thekeyif thekeyexists in the cache. Otherwise, returns-1.void put(int key, int value)Update the value of thekeyif present, or inserts thekeyif not already present. When the cache reaches itscapacity, it should invalidate and remove the least frequently used key before inserting a new item. For this problem, when there is a tie (i.e., two or more keys with the same frequency), the least recently usedkeywould be invalidated.
To determine the least frequently used key, a use counter is maintained for each key in the cache. The key with the smallest use counter is the least frequently used key.
When a key is first inserted into the cache, its use counter is set to 1 (due to the put operation). The use counter for a key in the cache is incremented either a get or put operation is called on it.
The functions get and put must each run in O(1) average time complexity.
Example 1:
Input
["LFUCache", "put", "put", "get", "put", "get", "get", "put", "get", "get", "get"]
[[2], [1, 1], [2, 2], [1], [3, 3], [2], [3], [4, 4], [1], [3], [4]]
Output
[null, null, null, 1, null, -1, 3, null, -1, 3, 4]
Explanation
// cnt(x) = the use counter for key x
// cache=[] will show the last used order for tiebreakers (leftmost element is most recent)
LFUCache lfu = new LFUCache(2);
lfu.put(1, 1); // cache=[1,_], cnt(1)=1
lfu.put(2, 2); // cache=[2,1], cnt(2)=1, cnt(1)=1
lfu.get(1); // return 1
// cache=[1,2], cnt(2)=1, cnt(1)=2
lfu.put(3, 3); // 2 is the LFU key because cnt(2)=1 is the smallest, invalidate 2.
// cache=[3,1], cnt(3)=1, cnt(1)=2
lfu.get(2); // return -1 (not found)
lfu.get(3); // return 3
// cache=[3,1], cnt(3)=2, cnt(1)=2
lfu.put(4, 4); // Both 1 and 3 have the same cnt, but 1 is LRU, invalidate 1.
// cache=[4,3], cnt(4)=1, cnt(3)=2
lfu.get(1); // return -1 (not found)
lfu.get(3); // return 3
// cache=[3,4], cnt(4)=1, cnt(3)=3
lfu.get(4); // return 4
// cache=[4,3], cnt(4)=2, cnt(3)=3
Constraints:
0 <= capacity <= 1040 <= key <= 1050 <= value <= 109- At most
2 * 105calls will be made togetandput.
class LFUCache {
private final Map<Integer, Node> map;
private final Map<Integer, DoublyLinkedList> freqMap;
private final int capacity;
private int minFreq;
public LFUCache(int capacity) {
this.capacity = capacity;
map = new HashMap<>(capacity, 1);
freqMap = new HashMap<>();
}
public int get(int key) {
if (capacity == 0) {
return -1;
}
if (!map.containsKey(key)) {
return -1;
}
Node node = map.get(key);
incrFreq(node);
return node.value;
}
public void put(int key, int value) {
if (capacity == 0) {
return;
}
if (map.containsKey(key)) {
Node node = map.get(key);
node.value = value;
incrFreq(node);
return;
}
if (map.size() == capacity) {
DoublyLinkedList list = freqMap.get(minFreq);
map.remove(list.removeLast().key);
}
Node node = new Node(key, value);
addNode(node);
map.put(key, node);
minFreq = 1;
}
private void incrFreq(Node node) {
int freq = node.freq;
DoublyLinkedList list = freqMap.get(freq);
list.remove(node);
if (list.isEmpty()) {
freqMap.remove(freq);
if (freq == minFreq) {
minFreq++;
}
}
node.freq++;
addNode(node);
}
private void addNode(Node node) {
int freq = node.freq;
DoublyLinkedList list = freqMap.getOrDefault(freq, new DoublyLinkedList());
list.addFirst(node);
freqMap.put(freq, list);
}
private static class Node {
int key;
int value;
int freq;
Node prev;
Node next;
Node(int key, int value) {
this.key = key;
this.value = value;
this.freq = 1;
}
}
private static class DoublyLinkedList {
private final Node head;
private final Node tail;
public DoublyLinkedList() {
head = new Node(-1, -1);
tail = new Node(-1, -1);
head.next = tail;
tail.prev = head;
}
public void addFirst(Node node) {
node.prev = head;
node.next = head.next;
head.next.prev = node;
head.next = node;
}
public Node remove(Node node) {
node.next.prev = node.prev;
node.prev.next = node.next;
node.next = null;
node.prev = null;
return node;
}
public Node removeLast() {
return remove(tail.prev);
}
public boolean isEmpty() {
return head.next == tail;
}
}
}type LFUCache struct {
cache map[int]*node
freqMap map[int]*list
minFreq int
capacity int
}
func Constructor(capacity int) LFUCache {
return LFUCache{
cache: make(map[int]*node),
freqMap: make(map[int]*list),
capacity: capacity,
}
}
func (this *LFUCache) Get(key int) int {
if this.capacity == 0 {
return -1
}
n, ok := this.cache[key]
if !ok {
return -1
}
this.incrFreq(n)
return n.val
}
func (this *LFUCache) Put(key int, value int) {
if this.capacity == 0 {
return
}
n, ok := this.cache[key]
if ok {
n.val = value
this.incrFreq(n)
return
}
if len(this.cache) == this.capacity {
l := this.freqMap[this.minFreq]
delete(this.cache, l.removeBack().key)
}
n = &node{key: key, val: value, freq: 1}
this.addNode(n)
this.cache[key] = n
this.minFreq = 1
}
func (this *LFUCache) incrFreq(n *node) {
l := this.freqMap[n.freq]
l.remove(n)
if l.empty() {
delete(this.freqMap, n.freq)
if n.freq == this.minFreq {
this.minFreq++
}
}
n.freq++
this.addNode(n)
}
func (this *LFUCache) addNode(n *node) {
l, ok := this.freqMap[n.freq]
if !ok {
l = newList()
this.freqMap[n.freq] = l
}
l.pushFront(n)
}
type node struct {
key int
val int
freq int
prev *node
next *node
}
type list struct {
head *node
tail *node
}
func newList() *list {
head := new(node)
tail := new(node)
head.next = tail
tail.prev = head
return &list{
head: head,
tail: tail,
}
}
func (l *list) pushFront(n *node) {
n.prev = l.head
n.next = l.head.next
l.head.next.prev = n
l.head.next = n
}
func (l *list) remove(n *node) {
n.prev.next = n.next
n.next.prev = n.prev
n.next = nil
n.prev = nil
}
func (l *list) removeBack() *node {
n := l.tail.prev
l.remove(n)
return n
}
func (l *list) empty() bool {
return l.head.next == l.tail
}use std::cell::RefCell;
use std::collections::HashMap;
use std::rc::Rc;
struct Node {
key: i32,
value: i32,
freq: i32,
prev: Option<Rc<RefCell<Node>>>,
next: Option<Rc<RefCell<Node>>>,
}
impl Node {
fn new(key: i32, value: i32) -> Self {
Self {
key,
value,
freq: 1,
prev: None,
next: None,
}
}
}
struct LinkedList {
head: Option<Rc<RefCell<Node>>>,
tail: Option<Rc<RefCell<Node>>>,
}
impl LinkedList {
fn new() -> Self {
Self {
head: None,
tail: None,
}
}
fn push_front(&mut self, node: &Rc<RefCell<Node>>) {
match self.head.take() {
Some(head) => {
head.borrow_mut().prev = Some(Rc::clone(node));
node.borrow_mut().prev = None;
node.borrow_mut().next = Some(head);
self.head = Some(Rc::clone(node));
}
None => {
node.borrow_mut().prev = None;
node.borrow_mut().next = None;
self.head = Some(Rc::clone(node));
self.tail = Some(Rc::clone(node));
}
};
}
fn remove(&mut self, node: &Rc<RefCell<Node>>) {
match (node.borrow().prev.as_ref(), node.borrow().next.as_ref()) {
(None, None) => {
self.head = None;
self.tail = None;
}
(None, Some(next)) => {
self.head = Some(Rc::clone(next));
next.borrow_mut().prev = None;
}
(Some(prev), None) => {
self.tail = Some(Rc::clone(prev));
prev.borrow_mut().next = None;
}
(Some(prev), Some(next)) => {
next.borrow_mut().prev = Some(Rc::clone(prev));
prev.borrow_mut().next = Some(Rc::clone(next));
}
};
}
fn pop_back(&mut self) -> Option<Rc<RefCell<Node>>> {
match self.tail.take() {
Some(tail) => {
self.remove(&tail);
Some(tail)
}
None => None,
}
}
fn is_empty(&self) -> bool {
self.head.is_none()
}
}
struct LFUCache {
cache: HashMap<i32, Rc<RefCell<Node>>>,
freq_map: HashMap<i32, LinkedList>,
min_freq: i32,
capacity: usize,
}
/**
* `&self` means the method takes an immutable reference.
* If you need a mutable reference, change it to `&mut self` instead.
*/
impl LFUCache {
fn new(capacity: i32) -> Self {
Self {
cache: HashMap::new(),
freq_map: HashMap::new(),
min_freq: 0,
capacity: capacity as usize,
}
}
fn get(&mut self, key: i32) -> i32 {
if self.capacity == 0 {
return -1;
}
match self.cache.get(&key) {
Some(node) => {
let node = Rc::clone(node);
self.incr_freq(&node);
let value = node.borrow().value;
value
}
None => -1,
}
}
fn put(&mut self, key: i32, value: i32) {
if self.capacity == 0 {
return;
}
match self.cache.get(&key) {
Some(node) => {
let node = Rc::clone(node);
node.borrow_mut().value = value;
self.incr_freq(&node);
}
None => {
if self.cache.len() == self.capacity {
let list = self.freq_map.get_mut(&self.min_freq).unwrap();
self.cache.remove(&list.pop_back().unwrap().borrow().key);
}
let node = Rc::new(RefCell::new(Node::new(key, value)));
self.add_node(&node);
self.cache.insert(key, node);
self.min_freq = 1;
}
};
}
fn incr_freq(&mut self, node: &Rc<RefCell<Node>>) {
let freq = node.borrow().freq;
let list = self.freq_map.get_mut(&freq).unwrap();
list.remove(node);
if list.is_empty() {
self.freq_map.remove(&freq);
if freq == self.min_freq {
self.min_freq += 1;
}
}
node.borrow_mut().freq += 1;
self.add_node(node);
}
fn add_node(&mut self, node: &Rc<RefCell<Node>>) {
let freq = node.borrow().freq;
match self.freq_map.get_mut(&freq) {
Some(list) => {
list.push_front(node);
}
None => {
let mut list = LinkedList::new();
list.push_front(node);
self.freq_map.insert(node.borrow().freq, list);
}
};
}
}
/**
* Your LFUCache object will be instantiated and called as such:
* let obj = LFUCache::new(capacity);
* let ret_1: i32 = obj.get(key);
* obj.put(key, value);
*/